The present invention generally relates to a pulse damper for a liquid chromatographic solvent delivery system and, in particular, relates to such a pulse damper having an elastomer as a compliant medium.
In the analysis of a sample via liquid chromatography the sample solution is injected into a solvent stream which carries the sample through a chromatographic separating column. Conventionally, the solvent is introduced into a piston cylinder, for example, by the withdrawal stroke of a piston, from a solvent reservoir and delivered to an injector and column by the extension stroke of the piston. For reasons well known in the art, sharp pressure changes, i.e., pulses, occur between each piston cycle, which pulses are detrimental to the chromatographic analysis. Consequently, considerable time and effort has been expended to reduce the sharpness of these pressure changes. As a result, most single piston pumps in liquid chromatography instruments include a pulse damper serially connected in the solvent delivery conduit.
Conventional pulse dampers store pressure via a spring or by a compressible fluid or gas, usually air or methanol. In the case of a fluid or gas the compliant medium is stored in a reservoir, and, being confined, is pressure absorbent and thus reduces the magnitude of the solvent pressure variations. Typical pulse dampers utilize either a metal or a PTFE (polytetrafluroethelene) diaphragm to isolate the solvent from the compliant medium.
These pulse dampers have a number of disadvantages, the major one of which is that the compliant medium must be completely sealed, i.e., no leakage whatsoever thereof is permitted. If any leakage occurs the volume allowed for solvent flow increases and subsequently overstretches the diaphragm resulting in the rupture thereof.
Another disadvantage is the precision required in loading the compliant medium. For instance, in liquid chromatography it is preferred that the solvent clearance volume be small. However, if this volume is too small, thermal expansion of the compliant medium expands the diaphragm into the solvent ports, thereby rupturing the diaphragm. If the solvent clearance volume is too large, the volume limits the solvent composition transition volume. In addition, because the solvent clearance is small and the diaphragm is quite close to the solvent ports, if the solvent pressure is abruptly relieved a shockwave occurs in the compliant medium which often forces the diaphragm to rupture against a port.
Yet another disadvantage, which is particularly acute when methanol is the compliant medium, is that methanol swells conventionally compatible "O-ring" materials and thus requires the use of a more expensive spring loaded PTFE seal which includes a metal spring. However, the use of such a seal in conjunction with the typical aluminum housing causes electrolytic pitting of the aluminum and hence forces the use of a more expensive stainless steel housing.
Finally, any failure requires an elaborate refilling procedure to refurbish the failed unit.